preccx.1

編譯器的辭法分析器工具

libcc.a has been placed.
.PP
Note that `\\x\0{@ $x @}' has no real effect, so it has been dropped
from most of the points in the script where it might have been expected.
.PP
Here is the same script, but suitably coded for versions of
\fIpreccx\fP up to 2.40.
.IP
# define TOKEN char
.IP
# define VALUE int
.IP
# define BEGIN call_mode=0;printf("\\nready> ");
.IP
#\0include\0"cc.h"
.IP
#\0include\0<ctype.h>
.IP
static int acc;
.IP
@\0digit\0=\0(isdigit)\0\0\0\0\0\0{: $$=$1-'0';
.IP
@\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0acc=acc*10+$1;:}
.IP
@\0posint=\0digit\0posint\0\0\0{:$$=$2; :}
.IP
@\0\0\0\0\0\0\0|\0digit\0\0\0\0\0\0\0\0\0\0{: $$=$1;acc=0; :}
.IP
@\0int\0\0\0=\0<'-'>\0posint\0\0\0{: $$=-$2; :}
.IP
@\0\0\0\0\0\0\0|\0posint
.IP
@\0atom\0\0=\0<'('>expr<')'>\0{: $$=$2; :}
.IP
@\0\0\0\0\0\0\0|\0int
.IP
@\0expr\0\0=\0atom\0sign_sum\0\0{: $$=$1+$2; :}
.IP
@\0\0\0\0\0\0\0|\0atom
.IP
@\0sign_sum=\0<'-'>\0atom\0sign_sum
.IP
@\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0{: $$=-$1+$3; :}
.IP
@\0\0\0\0\0\0\0\0\0|\0<'-'>\0atom\0\0\0{: $$=-$2; :}
.IP
@\0\0\0\0\0\0\0\0\0|\0<'+'>\0atom\0sign_sum
.IP
@\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0{: $$=$1+$3; :}
.IP
@\0\0\0\0\0\0\0\0\0|\0<'+'>\0atom\0\0\0{: $$=$2; :}
.IP
@\0top\0\0\0\0\0=\0expr\0\0\0\0\0\0\0\0\0{: printf("=%d\\n",$1); :}
.IP
MAIN(top)
.PP
For an example of a parser which uses parameters essentially, the following
definition of a parser which accepts only the fibonacci sequence as
input may be useful:
.IP
# define TOKEN char
.IP
# define VALUE char*
.IP
# include "ccx.h"
.IP
# include <math.h>
.IP
# define INT(x) (int)(x)
.IP
# define DIV(m,n) INT(INT(m)/INT(n))
.IP
# define MOD(m,n) INT(INT(m)%INT(n))
.IP
# define LOG10(n) INT(log10((double)(n)))
.IP
# define DBLE(n)  (double)(n)
.IP
# define TEN      DBLE(10)
.IP
#\0define\0FIRSTDIGIT(n)\0\\
(0!=n)?DIV((n),pow(TEN,DBLE(LOG10(n)))):0
.IP
#\0define\0LASTDIGITS(n)\0\\
(0!=n)?MOD((n),pow(TEN,DBLE(LOG10(n)))):0
.IP
MAIN(fibber)
.IP
\0@fibber\0\0\0=\0{\0fibs\0$!\0}*
.IP
\0@fibs\0\0\0\0\0=\0fib((PARAM)1,(PARAM)1)\k
.IP
\0@\0\0\0\0\0\0\0\0\0\0\0{:\0printf("%d\0terms\0OK\\n",(int)$k);\0:}
.IP
\0@fib(a,b)\0=\0number(a)\0<','>\0fib(b,a+b)\k\0{@\0$k+1\0@}
.IP
\0@\0\0\0\0\0\0\0\0\0|\0<'.'>\0<'.'>\0\0\0\0\0{@\00\0@}
.IP
\0@\0\0\0\0\0\0\0\0\0\0{:\0printf("Next\0terms\0are\0%d,%d,..\\n",(int)a,(int)b);\0:}
.IP
\0@number(n)=\0digit(n)
.IP
\0@\0\0\0\0\0\0\0\0\0|\0digit(FIRSTDIGIT(n))\0number(LASTDIGITS(n))
.IP
\0@digit(n)\0=\0<n+'0'>\0\0/*\0rep.\0of\01\0digit\0n\0*/
.PP
The\0following\0are\0some\0example\0inputs\0and\0responses:
.IP
1,1,2,3,5,..\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0
Next\0terms\0are\08,13,..\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0
5\0terms\0OK\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0
.IP
1,1,2,3,5,8,13,21,34,51,85,..\0\0\0\0\0\0\0\0\0\0\0\0\0
error:\0failed\0parse:\0probable\0error\0at\0<>1,85,..\0

.SH\0FILES
The following files are to be found in the \fI/users/news/preccx\fP directory:
.PD 0
.TP 15
preccx
Preccx executable
.TP 15
preccx.y
Preccx definition in its own language
.TP 15
lex.y
Tokenizer for \fIpreccx\fP
.TP 15
c.y
C parser for \fIpreccx\fP
.TP 15
preccx.c
Preccx C source code (generated by \fIpreccx\fP from \fIpreccx\fP.y).
.TP 15
preccx.h
Preccx header file, needed only to construct \fIpreccx\fP.
.TP 15
preamble.c
Auxiliary functions, needed only to construct \fIpreccx\fP.
.TP 15
preamble.h
Header file for preamble.c, needed only to construct \fIpreccx\fP.
.TP 15
common.c
Simple parsers common to both non-parameterised and parameterised parser
kernels. Needed to make \fIcommon.o\dP, included in libcc.a.
.TP 15
engine.c
Runtime engine. Needed to make \fIengine.o\dP, included in libcc.a.
.TP 15
ccx.c
The source code of the \fIpreccx\fP 1.0 kernel operations, needed to
make \fIccx.o\fP, included in libcc.a.
.TP 15
cc.c
The source code of the unparameterized \fIpreccx\fP 1.0 kernel operations,
needed to make \fIcc.o\fP, included in libcc.a.
.TP 15
ccx.h
The header file of the \fIpreccx\fP parameterized kernel operations, needed
by codes generated by \fIpreccx\fP.
.TP 15
cc.h
The header file of the unparameterized \fIpreccx\fP kernel operations,
an alternative to ccx.h if you do not use parameterized definitions.
.TP 15
yystuff.c
Default lexer which allows you to escape newlines.
.TP 15
on_error.c
Default error routines.
.TP 15
atexit.c
In case atexit() is not present on your system.
.TP 15
libcc.a
The library containing cc.o, ccx.o and yystuff.o, needed to compile an
executable from code built by \fIpreccx\fP.
.TP 15
Makefile
The makefile for \fIpreccx\fP.
.TP 15
test.y
Simple test script for \fIpreccx\fP.
.TP 15
test.c
C output from the \fItest.y\fP script.
.TP 15
test
The test parser built by `gcc \-ansi \-o test test.c \-L ... \-lcc'.
.PD
.SH "SEE ALSO"
yacc(1),
lex(1),
gcc(1L),
.SH AUTHOR
Peter Breuer, Programming Research Group,
Oxford University Computing Laboratory, UK.
.br
Man page also hacked by Jonathan Bowen.
.SH BUGS
1. On Sun3's, the \fIgcc\fP compiler still complains that \fIprintf\fP
is being redefined.  I don't know why. If anyone finds the right
compiler switch to magic this away, please tell me! For the hp300
series, the switch is \-D__hp9000s300, if that's any clue?
.PP
2. (Cured Mar 10 1992 in v1.1)
.PP
3. If you drastically change the type of VALUE in your
script (make it larger than char*), you will also have to change the type
declared in cc.h and then recompile the libcc.a library. This is not a bug
but a feature.
.PP
4. (Cured Mar 17 1992 in v1.2).
.PP
5. It has been reported that the IBM `ANSI' C compiler does not like the
.IP
typedef STATUS PARSER();
.PP
definition made by \fIpreccx\fP. That is their problem.
.PP
7. (patch issued for \fIpreccx\fP 2.30+ April 15 1993). Error in p_uniq0
code prevented recognition of all backtrack errors, with the effect
that they were caught as failed parses some time later instead.
.PP
8. (patch issued for 2.40+ July 1994). \fIPreccx\fP's C expresions don't
permit the use of `.' as an operator. My omission. Use a macro
instead until corrected (corrected).
.PP
Please report problems to \fI<[e-mail]>\fP.
.SH NOTES
In version 1.30 and above, script lines can be continued by placing an `@'
at the beginning of the next line, without a `\\' at the end of the
previous line. Each sequence of `@' continued lines must be terminated
by an empty line.
.PP
Version 1.40 introduced a hook TOKEN *yybuffer for external lexical analysers.
This is where lexers must eventually write their output for \fIpreccx\fP to see
it. Version 2.0 and above use a special routine mygets() to call
yylex() which places the TOKEN returned by yylex() in the right place
automatically. For backwards compatibility, it is still possible to
write into yybuffer directly, however. Note that, as mentioned already,
EOF is tested by looking at the global int yytchar.
The default yylex() lexer in libcc.a handles all this correctly.
 Version 2.41 and above adds a  special call get1token() which is used
in mygets() to get one  token from yylex(). You can use it to skip a
token by  calling  it from an error handler instead of within mygets().
All  calls to the lexer now go through get1token() and all interactions
with the buffer go through get1token() and  realignbuffer().
.PP
The default zer_error() handler supplied with preccx simply prints an error
message and the unparsed portion of the string. That might well be
\fIall\fP of the string, since \fIpreccx\fP parsers try their darn'dest
to make a match, then backtrack, so the (TOKEN *)maxp pointer is provided. This
points to the deepest successful penetration into the incoming string, and is
usually the point to look for the error. The pointer (TOKEN *)pstr shows
the unparsed string, of which (TOKEN *)maxp will be an end-segment (the
last TOKEN, in fact).
.PP
If you want to try and resync the parse at an error, a sensible thing to
do would be to (rewrite zer_error to) skip a token at maxp, and rerun the parse.
You will have to read the code of the run() function defined in \fBcc.c\fP to
make sense of it, but you might try:
.IP
strcpy(maxp,maxp+1);tok=the_top_level_parser();
.IP
if(GOODSTATUS(tok)){
.IP
pc=0;pc=p_evaluate());
.IP
}
.IP
else\0printf("At\0least\0I\0tried!\\n"));
.PP
Using a counter to set a maximal number of resync attempts in a single line
would also be sensible!
.PP
You can obviate any bad_error() call by
making sure that the top-level parser has a failsafe fallthrough to
a ?* parser, with some kind of error action attached.
.PP
The version 2.x series \fIpreccx\fP extended version 1.x by allowing
parameters to each clause of the grammar (i.e., it treats inherited
attribute grammars as well as synthetic ones), and by introducing the
`!' (cut) marker. This can be inserted in expressions in order to stop
backtracking through that point, which is useful in avoiding
excessively long searches for alternate parses when no alternate is
possible.
.PP
Promises: version 2.x will eventually eliminate the archaic
\fIyacc\fP-style of stack manipulation with something much nicer
(achieved in 2.4x series). Version 3.0 should implement tight
type-checking. Contact the author for the most recent version.